Try this code.
Update myTable set MyColumn = NULL
The above query will set the entire column to null as per the question's title (i.e. To set a column value to NULL in SQL Server Management Studio).
You can use this query, to set the specific row on a specific column to null this way:
Update myTable set MyColumn = NULL where Field = Condition.
Here, the above code will set the specific cell to null as per the inner question (i.e. To clear the value from a cell and make it NULL).